Primarily, homework is still around because it's an entrenched concept that's been around for centuries. The longer anything has been around a certain way, the more difficult it is to create change.
I am reminded of the story of a young bride, whose husband asked why she was cutting the end from a roast before cooking. Her reply was, "Well, that's how my mother always did it." So, they called up the mom, and asked; she had the same exact response as her daughter.
Her grandma was still living, so they all asked her. Grandma said, " Because that's the only way it could fit into the pan I had!"
So, even at the personal level, old habits die hard. At the level of national public institutions, multiply that factor exponentially!